"Nothing in biology makes sense, except for in the light of evolution."
Viruses exist, because they can exist. The fact that virus DNA is often quite similar to the DNA of the creature it infects suggests that viruses may have started as little more than fragments of DNA that became seperated from it's original cell (say the cell was destroyed, and the DNA shattered a bit). Because the DNA fragments still container markers that allow it to align with the DNA in a whole cell, the rules of molecular biology apply, and the fragment could interact with the DNA in a complete cell.
The vast majority of fragments would either be inert (some viruses are so close to the original DNA they do nothing more than self replicate, without affecting the host), do nothing (missing gene markers so the fragment can't align properly with a complete strand), or most likely never come in contact with compatable DNA. On the other hand, the fragments that could interfere with the DNA in a complete cell and cause that cell to replicate that fragment would do so. To assume the virus has some sort of purpose or motive is folly. The virus is just a molecule, doing what molecules do. Because the laws of physics provided the DNA fragments with the potential to reproduce - and because the fragments do contain genes that can change from one generation to the next - natural selection came in to play - and the strains that exist today are the ones that are good at infecting and reproducing.
Viruses could very well be an unavoidable by-product of the way our celluar biology works. (To claim it's "Mother Nature's" defense against aliens is rediculous. Most viruses can only infect a specific species. Furthermore, to assume that other lifeforms would be both carbon based AND employ the same molecular chemistry for individual genes and gene markers is quite a long shot.)
- Lime